This is the first lecture of the series on respiratory mechanics presented to the Critical Care Medicine fellows at Cleveland Clinic. This lecture is split in two parts and Part B will be uploaded next week.
4 pressures at specific locations:
(1.) Airway pressure (Paw) or Pressure at the airway opening (Pao):
– This is the pressure measured by the ventilator in a mechanically ventilated patient
– In a spontaneously breathing patient, Paw/Pao = zero (mouth opening is exposed to atmospheric pressure)
(2.) Alveolar pressure (Palv):
– Pressure within an alveolus.
– Practically not feasible to have a catheter small enough to sit inside an alveolus to directly measure Palv.
– However, there is no pressure differential between airway opening and alveolus. Hence, Pao becomes a reflection of Palv.
– E.g. plateau pressure (end-inspiratory occlusion) and total PEEP (end-expiratory occlusion) are reflectors of Palv at end-inspiration and end-expiration respectively.
(3.) Pleural pressure (Ppl):
– Pressure within the pleural space.
– Can be estimated by using esophageal balloon.
(4.) Body surface pressure (Pbs):
– Pressure outside the body at the body surface.
– Typically equals atmospheric pressure and hence zero.
4 pressures across specific locations:
(1.) Transairway pressure:
– Pao minus Palv.
– Represents ‘resistive pressure’ in the presence of airflow.
– In the absence of airflow, transairway pressure = zero (Hence, Pao = Palv); see above.
(2.) Transalveolar pressure:
– Palv minus Ppl.
– It is the transmural pressure of the alveolus/lung parenchyma.
– Alveolar inflation is proportional to transalveolar pressure.
– Transalveolar pressure (not Palv) is the true measure of alveolar stress.
(3.) Transchestwall pressure:
– In the absence of muscle activity (Pmus): transchest wall pressure = Ppl – Pbs
– It is the transmural pressure of the chest wall.
(4.) Transpulmonary pressure:
– Transpulmonary pressure = Pao minus Ppl = Transairway pressure minus transalveolar pressure
– In the absence of airflow, transairway pressure becomes zero. Hence, in this state, transpulmonary pressure reflects transalveolar pressure.
– Clinical utility:
(a) End-inspiratory transpulmonary pressure = Plateau pressure minus end-inspiratory Ppl
This reflects end-inspiratory transalveolar pressure, and hence, end-inspiratory alveolar stress.
(b) End-expiratory transpulmonary pressure = total PEEP minus end-expiratory Ppl
This reflects end-expiratory transalveolar pressure, and hence, end-expiratory alveolar stress.
– In simple words, plateau pressure and PEEP and not true indicators of alveolar stress. Both have to be substracted by pleural pressure (the extramural pressure of the alveolus) to get a better idea of alveolar stress.